The present invention relates to apparatus and methods for eliminating or substantially inhibiting electrostatic deposition of charged particles from the coolant onto the surface of an inlet-mixer of a jet pump forming part of a water recirculation system in a boiling water nuclear reactor, and for inhibiting stress corrosion cracking of the metallic parts. This invention particularly relates to an insulating barrier coating that eliminates or substantially inhibits the interaction between the conductive metal housing of the inlet-mixer of the jet pump assembly and the ionic particles in the fluid.
In a boiling water nuclear reactor, an annular space is defined between the core shroud and the reactor pressure vessel wall. Jet pumps are located in the annular space for recirculating coolant through the reactor. The recirculation system circulates the cooling medium around the nuclear reactor core. Jet pumps, which contain no moving parts, provide an internal circulation path for the core coolant flow. Typically, a substantial number of jet pumps, for example, on the order of sixteen to twenty-four, are installed in this annular space. Each jet pump assembly consists of a riser assembly, a riser brace, two inlet-mixer assemblies, and two diffuser assemblies. The inlet-mixer includes a nozzle and a suction inlet. The nozzle may have one orifice or five orifices, depending on the jet pump design. The top of the inlet-mixer is mechanically clamped to the top of the riser transition piece, while the exit end of the inlet-mixer fits into a slip joint with the top of the diffuser. The inlet-mixer is a removable component.
A recirculation pump, external to the reactor vessel, pulls suction from the downward flow of coolant in the annular space. The coolant is pumped to a higher pressure, and is distributed through a manifold to the jet pumps, where the coolant flows in an upward direction through the risers. The coolant splits in the transition piece and changes direction. It is then accelerated in a downward direction through the nozzles and into a mixer section of the jet pump. The nozzles cause a high velocity coolant flow that is approximately one third of the core flow and discharge into the inlet-mixers. Momentum causes surrounding water in the downcomer region of the annulus to also enter the mixer section where it mixes with the outflow from the nozzles for flow through the mixer section and diffuser. This combined flow discharges into the lower core plenum. The coolant then flows upward between the control rod drive guide tubes and is distributed for flow along individual fuel rods inside the fuel channels.
Over time, contaminants build up on the inside surface of the inlet-mixers including the jet pump nozzles, forming a layer of xe2x80x9ccrud.xe2x80x9d There is also potential for stress corrosion cracking along these surfaces. The build-up of xe2x80x9ccrudxe2x80x9d is believed caused by charged particles suspended in the coolant which interact with the metallic inner surface of the inlet-mixer inducing a triboelectrostatic charge on the surface. This charge creates an electrostatic potential that attracts the suspended particles in the fluid to the metallic surface where they form a layer of particle contaminants. The greatest deposition of xe2x80x9ccrudxe2x80x9d is observed in areas that experience a high velocity flow rate.
In the event that the build-up layer becomes excessive, the performance of the recirculation system will be degraded. This degradation will affect the efficiency of the plant because the recirculation pumps must be run at a higher speed to maintain core flow. Degradation of jet pump performance can also result in extreme jet pump vibration and damage to jet pump components. Eventually, the inlet-mixer must be mechanically cleaned or replaced during regular maintenance and refueling outages. This process is expensive and time consuming. Consequently, it is important that the layer of xe2x80x9ccrudxe2x80x9d be eliminated, substantially minimized or its rate of xe2x80x9cbuild upxe2x80x9d curtailedxe2x80x9d and that a clear flow path is maintained.
In the past, cleaning processes have been proposed that remove the xe2x80x9ccrudxe2x80x9d layer from the inside surface of the inlet-mixer. These processes require removal of the inlet-mixer from the reactor for cleaning in the fuel pool. This is typically accomplished at regular scheduled shutdowns of the reactor, at which times the necessary maintenance is performed. A process using an electrical circuit has also been proposed that minimizes the electrostatic deposition of charged particles on the inlet-mixer surfaces that are exposed to the free stream electrical potential (See U.S. Pat. No. 5,444,747). This process employs a DC circuit with an active element feedback loop that adjusts the surface potential of the inlet-mixer to minimize the net flux to the inner conducting surface of the parts and therefore minimizes particulate deposition. This process, however, requires significant attention and maintenance.
Accordingly, there remains a need for apparatus and methods of protecting the inlet-mixers of the jet pumps from contaminant build-up. Furthermore, there remains a need for a solution to the problem of xe2x80x9ccrudxe2x80x9d build-up which gradually degrades their performance and requires the need for periodically cleaning and maintaining the jet pump.
In accordance with a preferred embodiment of the present invention, there is provided an insulator coating for application to inlet-mixer surfaces to reduce the electrostatic potential between the ionic fluid flow and such surfaces and thereby eliminate or reduce the build-up of crud. Without the electrostatic potential, the particulate layer does not form or is substantially inhibited or reduced in its formation. Furthermore, the insulator coating reduces the susceptibility of the inlet-mixer surfaces to stress corrosion cracking by lowering the electrochemical potential (ECP).
To accomplish the foregoing and in accordance with a preferred embodiment of this invention, the interior surfaces of the inlet-mixer are provided with a coating which reduces or eliminates the build-up of charged particles on those surfaces. Particularly, the interior surfaces of each inlet-mixer are coated with a dielectric material. The non-conductive coating electrically insulates the surfaces of these parts from the flow and therefore interferes with the electrostatic potential caused by the induced triboelectrostatic charge on the metallic inner surfaces of the inlet-mixer and hence the potential for interaction with charged particles suspended in the water. Thus, the coating eliminates or greatly inhibits interaction of the conductive metal housing and the ionic particles in the coolant. The charged particles are not attracted to the dielectric material in the same manner as they are to the metallic surfaces and therefore the potential contaminating particles pass through the inlet-mixer without or with minimal deposition on the interior surfaces of the inlet-mixer. The coating, therefore, reduces or eliminates the need for costly cleaning and maintenance of the jet pump and maintains the flow path clear of these potential contaminants.
The dielectric coating electrochemically isolates the metal surface from the reactor water. The coating retards diffusion of oxygen to the metal surfaces. This results in reduced susceptibility to stress corrosion cracking of the metallic parts.
The coating is preferably a ceramic coating, e.g., a coating formed of TiO2 or Ta2O5, although other coatings as described below may also be employed. The coating is applied by placing the nozzle assembly in a heated vacuum reactor vessel. Once the reaction conditions have been achieved, chemical precursors, e.g., Ti(OC2H5)4 of a Ta(OC2H5)5, are introduced into the system. These compounds thermally decompose on the surface of the parts, producing the dielectric coating and releasing several gases. The product is then cooled and installed in the nuclear reactor.
In a preferred embodiment according to the present invention, there is provided apparatus for minimizing or eliminating deposition of particulates in flow passages of a jet pump for a boiling water reactor comprising an inlet riser for receiving a flow of coolant, an inlet-mixer for receiving the coolant flow from the riser including a nozzle and a mixing section for receiving coolant flow from the nozzle and a flow of coolant induced from an annular space between a reactor vessel and a core shroud and a coating along interior wall surfaces of at least a portion of the inlet-mixer for eliminating or minimizing deposition of particulates from the coolant flow along the interior wall portions.
In a further preferred embodiment according to the present invention, there is provided a method for minimizing or eliminating deposition of particulates on interior wall surfaces defining a coolant flow passage in a jet pump for a nuclear reactor, comprising the steps of depositing a ceramic oxide material on the interior wall surfaces of one of a nozzle and a mixing section forming part of an inlet-mixer of the jet pump.